An industrial truck using a torque converter includes, corresponding to the clutch operating lever positions (forward movement/rearward movement/1st gear/2nd gear/3rd gear/4th gear), two groups of hydraulic clutches, namely, a group of forward/rearward movement hydraulic clutches (a forward movement hydraulic clutch and a rearward movement hydraulic clutch), and a group of speed gear hydraulic clutches (a 1st gear hydraulic clutch, a 2nd gear hydraulic clutch, a 3rd gear hydraulic clutch, and a 4th gear hydraulic clutch), so that the direction of travel and speed gear are set by the engagement of respective hydraulic clutches selected one from each group, thus effecting the change of direction and speed of the truck.
A publicly known clutch hydraulic mechanism for feeding hydraulic oil to these hydraulic clutches is provided with a control valve for selectively feeding hydraulic oil to each said hydraulic clutches in response to the operating signals from the clutch operating lever (forward movement/rearward movement/1st gear/2nd gear/3rd gear/4th gear), so that the direction change and gear shift of the truck are effected by selectively feeding hydraulic oil from the control valve to the hydraulic clutches.
Further, in this control valve, the clutch oil pressure increasing time (clutch oil pressure waveform) is changed according to each speed gear (1st gear/2nd gear/3rd gear/4th gear) to make gear shifts smooth. In effecting a gear shift operation, the clutch oil pressure for the speed gear hydraulic clutch before the gear shift is quickly brought back to zero and then the clutch oil pressure for the speed gear hydraulic clutch after the gear shift is gradually increased to complete the gear shift.
In the conventional transmission mechanism described above, however,
(1) there is a substantial amount of time needed from the time the clutch oil pressure for the speed gear hydraulic clutch before a gear shift is reduced to almost zero until the clutch oil pressure for the speed gear hydraulic clutch after the gear shift is increased to provide a sufficient torque to the truck, and
(2) when a gear shift operation is effected, the clutch oil pressure in the entire circuit momentarily drops, so that the forward or rearward movement hydraulic clutch that does not need to be turned on or off has its clutch oil pressure momentarily dropping, accompanied by a clutch slip, and then rising again, the slip retarding the recovery of torque after the gear shift. These two factors cause a loss in the output shaft torque during a gear shift operation, arousing a feeling of torque loss or a shock, and a feeling of uneasiness such as a feeling of reversing on an uphill slope, in an operator during the gear shift.
A publicly known example will be later described in detail using drawings.